1. Field of the Invention
The invention relates to the field of the optical measurement of electrical quantities. It concerns a fiber-optic sensor, comprising
(a) a light source; PA1 (b) a piezoelectric sensor element; PA1 (c) a first bimode fiber with an entrance end and an exit end, in which fiber the LP.sub.01 fundamental mode and the even LP.sub.11 mode can propagate, and which fiber is at least partially fixed to the sensor element so that a change in the dimension of the sensor element in an electric field leads to a change in length in the fiber; and PA1 means for measuring the field-dependent change in length of the fiber. PA1 (e) the light source is a multimode laser diode; PA1 (f) the measuring means comprise a second bimode fiber of the same type as the first; PA1 (g) the parameters of the two bimode fibers are selected so that the interference contrast is approximately equal to zero in each instance for the path differences of both modes in the individual bimode fibers and for the sum of the path differences; and PA1 (h) respective monomode fibers for the transmission of the light are provided between the light source and the entrance end of the first bimode fiber, and the exit end of the first bimode fiber and the entrance end of the second bimode fiber. PA1 (a) the second bimode fiber is at least partially fixed to a piezoelectric modulator; PA1 (b) two detectors for measuring the intensities of the two modes are disposed at the exit end of the second bimode fiber; PA1 (c) the output signals of the two detectors pass via a subtractor to the input of a quadrature regulator; PA1 (d) the output of the quadrature regulator controls the modulator; and PA1 (e) the output signal of the quadrature regulator is passed via a high pass filter to a signal output.
Such a fiber-optic sensor is known, for example, from EP-A1-0,433,824.
2. Discussion of Background
Fiber-optic sensors for the measurement of electric fields and voltages have already been described in various publications such as, for example, European Patent Applications EP-A1-0,316,619 and EP-A1-0,316,635 or the articles by K. Bobherr and J. Nehring in Appl. Opt. 27, pp. 4814-4818 (1988) or Opt. Lett. 14, pp. 290-292 (1989).
The measurement principle employed in this case is based on the inverse piezoelectric effect in materials with a selected crystal symmetry. The temporally periodic change in dimension which an appropriate piezoelectric body experiences in an alternating electric field is transmitted to a glass fiber fixed to the body.. The change in length of the fiber is then proportional to the field amplitude or voltage amplitude and is interferometrically measured and evaluated.
Various types of glass fiber interferometers may be employed for the interferometric measurement. On account of its simplicity, among these types the bimode fiber interferometer known from the article by B. Y. Kim etal., Opt. Lett. 12, pp. 729-731 (1987) is of particular interest. In this interferometer, the parameters of the sensor fiber are selected so that precisely two modes (the LP.sub.01 fundamental mode and the even LP.sub.11 mode) can propagate in the fiber.
In the bimode fiber interferometer, light is passed from a coherent light source, e.g. a laser diode, through a bimode fiber which is fixed to a piezoelectric sensor element for the electric field E. The two modes are excited by the light and propagate differently in the fiber. At the fiber end it is then possible to observe an interference pattern which arises from the superposition of these two modes. In this case, a change in length of the fiber leads to a differential phase shift between the two modes, which is expressed in a corresponding change of the interference pattern.
The two mutually adjacent substructures of the interference pattern are detected by two detectors (e.g. in the form of photodiodes). Two signals V.sub.11 and V.sub.12 which are phase-shifted by 180.degree. are present at their output: EQU V.sub.11 =(1/2)V.sub.0 (1+a*cos.PHI.(t)) (1) EQU V.sub.12 =(1/2)V.sub.0 (1-a*cos.PHI.(t)) (2)
with (t).PHI.=A*sin.OMEGA.t+.theta.(t). The phase shift .theta.(t) between the two modes is thus composed of a temporally periodic component A*sin.OMEGA.t generated by the alternating field to be measured (in this case, A is proportional to the amplitude of the field) and an arbitrary phase term .theta.(t) which may likewise change with time, e.g. in consequence of temperature-dependent fluctuations of the fiber length. Finally, V.sub.0 is proportional to the optical power and a is a measure of the interference contrast.
The target term A*sin.OMEGA.t is frequently obtained by a homodyne detection process from the output signals of the detectors (in the case of a fiber-optic sensor with a monomode fiber, see in this connection: D. A. Jackson etal., Appl. Opt. 19, pp. 2926-2929 (1980); a corresponding fiber-optic sensor with a bimode fiber is described in the European Application EP-A1-0,433,824 cited in the introduction). In this process, the sensor fiber is additionally guided via a piezoelectric modulator. By means of this modulator 4, the phase difference .PHI.(t) is set to +(pi/2) or -(pi/2) (modulo 2pi). To this end, the modulator is a component of a regulating circuit which consists of the detectors, a subtractor and a quadrature regulator and which sets correspondingly to zero the differential voltage EQU V=V.sub.11 -V.sub.12 =V.sub.0 *a*cos.PHI.(t) (3)
The two components A*sin.OMEGA.t and .theta.(t) of the phase shift are thus precisely balanced by the modulator by means of an appropriate (opposite) change in length of the fiber. The voltage present at the modulator then includes a slowly varying component which is proportional to .theta.(t) and a periodic component which is proportional to A*sin.OMEGA.t. The target component A*sin.OMEGA.t is filtered out by a high-pass filter and can be picked off at the signal output. As a result of this, the output signal is independent of the possible fluctuations of the laser intensity (i.e. V.sub.0) and of the interference contrast a.
In a series of practical applications of the sensor (e.g. in voltage measurement in outdoor substations), relatively large spacings may occur between the actual sensor head and the sensor electronic system (10 m to a few 100 m). It is inexpedient to bridge these spacings using the bimode fiber itself, since the influence of external disturbances (temperature fluctuations, mechanical vibrations etc.) increases correspondingly with increasing fiber length and the signal/noise ratio deteriorates. Rather, the light feed from the laser diode to the interferometer and the return guidance of the output signals of the interferometer should take place via separate glass fibers, which are not a component of the interferometer.
In the above-described homodyne process using an active phase modulator, it would however, be necessary to provide, in addition to the connecting glass fibers, an electrical connection between the sensor electronic system and the sensor head to drive the modulator. The attractiveness of a sensor operating with this type of interferometer would be very greatly restricted by this.
In two older German Patent Applications (file references P 41 14 253 5 and P 41 15 370 7) it has therefore been proposed in place of the known active signal detection, which requires an additional modulator in the measurement fiber with an appropriate electrical supply line, to provide a passive signal detection, which is based on the Guoy effect (in this connection, see: S. Y. Huang et al., Springer Proc. in Physics, Vol. 44 "Optical Fiber Sensors" , pp 38-43, Springer Verlag Berlin, Heidelberg (1989)), i.e. the phase difference between the interference patterns of the close field and remote field: in this case, the substructures of the close field and remote field (a total of 4) are separated in the sensor head by optical means and can be transmitted via separate glass fibers to a remote electronic evaluatlon system. In that system, the desired information on the change in length of the measurement fiber can be obtained by using at least three of these four substructures.
Using this proposed solution, a complete electrical separation is indeed achieved between sensor head and electronic evaluation system. However, this advantage is acquired in consideration of the fact that additional optical components (beam splitter) and a relatively complex electronic system must be employed. Over and above this, it is necessary to use a monomode laser diode, which necessitates special measures for the suppression of the light backscatter from the sensor into the diode.